Streamflow of 2006 - Water Year Summary

Introduction

The maps and graphs appearing in this summary describe streamflow
conditions for water-year 2006 (October 1, 2005 to September 30, 2006)
in the context of the 77-year period 1930-2006, unless otherwise noted.
The illustrations are based on observed data from the U.S. Geological
Survey's (USGS) National Streamflow Information Program. The period
1930-2006 was used because prior to 1930, the number of streamgages
was too small to provide representative data for computing statistics
for most regions of the country.

In the summary, reference is made to the term "runoff," which
is the depth to which a river basin, State, or other geographic area
would be covered with water if all the streamflow within the area
during a single year was uniformly distributed upon it. Runoff quantifies
the magnitude of water flowing through the Nation's rivers and streams
in measurement units that can be compared from one area to another.
The runoff value for a geographic area is computed as the median runoff
value for all streamgages in that geographic area. For example, the
runoff value for a state is the median for all streamgages in that
state, and the median for the Nation is the median value for all streamgages
in the Nation.

Each of the maps and graphs below can be expanded to a larger view
by clicking on the image. In all the graphics, a rank of 1 indicates
the highest flow of all years analyzed.

National Overview

Annual Median U.S. Runoff, 1930-2006

Runoff in the Nation’s rivers and streams during 2006 (10.17
inches) was below the long-term annual median for the United States
(11.93 inches). Nationwide, 2006 streamflow ranked 59 out of the
77 years in the period 1930-2006.

2006 Statewide Streamflow Ranks (1930-2006)

Above normal streamflow characterized most of the Pacific Coast
states, the Northeast states, and Hawaii. Below normal streamflow
was prevalent in the Southern Plains states and some Southeast
states. Some Northeast states recorded record high flow conditions
during 2006 compared to the historical period.

* Out of 68 years of historical data.
** Out of 63 years of historical data.

Regional Patterns

The United States (including Puerto Rico) is divided into 21 large
drainages, or water resources regions. These hydrologic areas are
based on surface topography and contain either the drainage area of
a major river, such as the Columbia, the combined drainage areas of
a series of rivers, such as the Texas-Gulf region which includes a
number of rivers draining into the Gulf of Mexico, or the area of
an island or island group. Water resources regions provide a coherent,
watershed-based framework for depicting streamflow variations.

In 2006, notable regional streamflow differences were observed
along the East Coast. The New England region, for example, recorded
its highest annual flow since 1930, reflecting the record high
precipitation that fell on this area during the year. The South-Atlantic
Gulf region, in contrast, was below normal. Below-normal conditions
also were prevalent for most the Great Plains regions and Alaska.

** Out of 63 years of historical data.

Seasonal Characteristics

Autumn (October - December 2005) Statewide Ranks

Autumn season (Oct-Dec) streamflow was normal nationwide (32nd highest
autumn flows in 77 years). Four states reported new record high autumn
streamflow: Maine, Vermont, New Hampshire, and Massachusetts.

* Out of 68 years of historical data.
** Out of 63 years of historical data.

Winter (January - March 2006) Statewide Ranks

Winter season (January-March) streamflow was at the low end of
normal for the Nation, ranking as the 53rd highest in 77 years.
Normal or above normal streamflow occurred across the northern
half of the conterminous U.S. and Hawaii, while below-normal flow
characterized most of the southern states and Alaska.

* Out of 68 years of historical data.
** Out of 63 years of historical data.

Spring (April - June 2006) Statewide Ranks

Spring season (April-June) streamflow was at the
low end of normal (56th highest in 77 years) nationwide. Similar
to the Winter season, Spring flows were normal or above normal
in the northern states and below normal in the southern states
and Alaska.

* Out of 68 years of historical data.
** Out of 63 years of historical data.

Summer (July - September 2006) Statewide Ranks

Summer season (July-September) streamflow was in
the normal range for the U.S., ranking 28th out of 77 summer seasons
in the record.

* Out of 68 years of historical data.
** Out of 63 years of historical data.

High and Low Flows

In any given month, on average, it is expected that five percent
of the streamgages will experience very high (>95th percentile)
and very low (<5th percentile) average streamflow. During water
year 2006, three months (October [2005], January, and June) had a
greater number of streamgages than expected reporting very high flows
(10, 14, and 7 percent respectively). In contrast, 6 months during
the water year had a greater number of streamgages than expected reporting
very low flows (February-April and June-August).

In 2006,
two percent of USGS streamgages reported new record high daily mean
streamflow. Since 1950, the largest number of streamgages reporting
new record highs in any one year was six percent, which occurred in
both 1965 and 1997.

The 7-day low flow is defined as the lowest average streamflow to
occur on seven consecutive days in a year. The number of new all-time
record 7-day low streamflows that have been set annually since
1999 (the year when the current period of widespread drought began
in the U.S.) has been well-below the number of records set during
previous multi-year drought periods, such as those of the 1950s and
1960s. In 1954, 1964, and 1977, for example, seven percent of the
streamgages set new all-time record 7-day low flows.

Additional Information

The USGS operates a network of nearly 7,400 streamgages nationwide,
many in real-time. Current information derived from these stations
is available on the web at http://water.usgs.gov/waterwatch.
Tables of data that summarize historical streamflow conditions by
State, beginning in the year 1900, can be accessed at http://water.usgs.gov/waterwatch/?m=statesum.
These tables are updated every few months to reflect the most current
streamflow data.

The streamflow information used to prepare this summary is also used
for water management, monitoring floods and droughts, bridge design,
and for many recreational activities. To obtain real-time and archived
streamflow data and information, visit http://water.usgs.gov/nwis.
Although the national streamgage network is operated primarily by
the USGS, it is funded by a partnership of 800 agencies at the Federal,
State, Tribal, and local levels. For more information about the streamgage
network, see http://water.usgs.gov/nsip/.